Dual-energy chest radiographs may be obtained either simultaneously or in rapid sequence with two x-ray exposures at different energies. Because of the non-linear nature of x-ray absorption in human body as a function of x-ray energy spectrum, dual-energy imaging techniques can be used to separate the image into two components: one primarily is composed of water (i.e., soft-tissue) and another primarily composed of calcium (i.e., bone). Due to the fact that the shadow of the ribs does not appear on the soft-tissue image, lung disease can be seen more readily on the soft-tissue image than in conventional chest x-rays.
FIG. 1 shows an example of how dual-energy radiography may be used to obtain chest images. In this example, two digital images, 11 and 12, have been acquired at different effective energy levels (here, 120 Kvp and 60 Kvp). These digital images have been supplied to a separation algorithm 13 that has, based on different x-ray absorption characteristics, produced a “standard” image 14, a bone equivalent image 15, and a soft-tissue equivalent image 16. In soft-tissue image 16, it can be seen that the shadows of the ribs and clavicles have been substantially removed.
However, dual-energy radiographs display the lungs and their structures with relatively low contrast due to higher image noise on the soft-tissue image. Consequently, some nodules can appear relatively indistinct and may be easier for a radiologist to detect on images from a conventional chest radiograph. Moreover, in some cases, mis-registration of the image pair used to form the bone and soft-tissue images can result in artifacts that obscure lung nodules.